1. Field of the Invention
This invention generally relates to a semiconductor device that includes a silicon nitride film having a high breakdown voltage and fabrication method therefor, a capacitive element and fabrication method therefor, and a MIS type semiconductor device and fabrication method therefor.
2. Description of the Related Art
A silicon nitride is often for use in a capacitor insulating film, passivation film, gate insulating film, and mask film in the selective process of a semiconductor device.
Silicon included in such silicon nitride film is supplied with mono-silane gas (SiH4), and nitrogen is supplied with ammonia gas (NH3) or nitrogen gas (N2). Then, the silicon nitride film is grown by vapor growth method with the use of a mixed gas that includes the afore-described gasses at appropriate ratios. In particular, plasma enhanced Chemical Vapor Deposition (plasma CVD) is commonly employed with the use of a plasma CVD apparatus, after the mixed gas is brought into a plasma state.
There is a demand that the silicon nitride film should have a high breakdown voltage used for the capacitor insulating film, passivation film, gate insulating film, and mask film in the selective process in order to ensure the reliability of the semiconductor device, especially which is used as a power device. Additionally, there is another demand that the silicon nitride film should be formed at low temperatures, since the electrode that lacks heat resistance is used in a compound semiconductor device suitable for the power device.
FIG. 1 is a graph showing thickness dependency of a breakdown voltage and capacitance of a capacitor that employs the silicon nitride film that has been grown in a conventional technique as a capacitor insulating film. The breakdown voltage of the capacitor becomes higher, in a rough proportion to the increased thickness of the silicon nitride film. However, the capacitance of the capacitor drastically decreases with the increased thickness of the silicon nitride film. That is to say, there is a trade-off relationship between the breakdown voltage of the capacitor and the dependency of the capacitance on the silicon nitride film.
The silicon nitride film that has been grown in the conventional technique does not sufficiently exhibit the breakdown voltage characteristic that satisfies the above-described demands. In order to obtain the capacitor (metal film-insulating film-metal film) having a desired breakdown voltage, for example, there is no other choice that the silicon nitride film sandwiched by the metal films are configured to have a thickness of twice the insulating film to enhance the breakdown voltage. Such thickened capacitor insulating film leads to a decrease in the capacitance of the capacitor, so the area of the capacitor has to be roughly doubled to ensure a given capacitance. However, if the area of the capacitor is increased, the chip size will also be increased. This is problematic in that it is difficult to realize a highly integrated semiconductor device and the costs thereof are increased.
It is known that hydrogen taken into the silicon nitrogen film prevents the breakdown voltage thereof from increasing.
FIG. 2 is a graph showing the relationship between hydrogen concentration and breakdown voltage that have been measured in the Fourier Transform Infrared technique (FTIR). As the hydrogen concentration taken into the film is increased, the breakdown voltage linearly decreases. Here, the breakdown voltage was measured where the silicon nitride has a thickness of 100 nm and a current density is 100 mA/mm2.
Japanese Patent Application Publication No. 9-260372 (hereinafter, referred to as Document 1) discloses a method of decreasing the hydrogen concentration in the film. In order to decrease the leakage current caused by the defect induced by hydrogen that has been taken into the silicon nitride film deposited by CVD, such deposited silicon nitride film is heated at high temperatures to emit hydrogen existent in the form of N—H or Si—H from the silicon nitride film as a gas. In this manner, the silicon nitride film having a low hydrogen concentration is obtainable.
Japanese Patent Application Publication No. 61-284929 (hereinafter, referred to as Document 2) discloses a method of forming the silicon nitride film with the use of mono-silane gas (SiH4), nitrogen gas (N2), and hydrogen gas (H2). The silicon nitride film is deposited by plasma CVD with approximately at least six percent of the flow rate of the hydrogen gas (hereinafter, referred to as hydrogen gas flow rate) relative to the overall gas flow rate. Japanese Patent Application Publication No. 5-47753 (hereinafter, referred to as Document 3) discloses a method of forming the silicon nitride film with the use of mono-silane gas (SiH4) and nitrogen gas (N2). The silicon nitride film is formed by plasma CVD with approximately five percent of the flow rate of the mono-silane gas (hereinafter, referred to as mono-silane gas flow rate) relative to the overall gas flow rate.
The method disclosed in Document 1, however, is supposed to be applied to a silicon semiconductor. Accordingly, after the silicon nitride film is formed, heat treatment is carried out at high temperatures of 800° C. or more. This makes it impossible to be applied to the compound semiconductor device that needs the processes implemented at relatively low temperatures. Besides, it is actually impossible to remove hydrogen completely as far as the technique that emits hydrogen from the silicon nitride film after the film is once formed is employed.
Accordingly, in order to obtain the silicon nitride film having a high breakdown voltage with the hydrogen concentration reduced to the minimum, there is a demand for a technique of forming the silicon nitride film in such a manner that hydrogen is not taken into the silicon nitride film during the film-forming process.
In addition, according to the methods disclosed in Document 2 and Document 3, as will be described later in detail, the hydrogen concentration in the silicon nitride film that has been formed cannot be decreased at low temperatures to be used in the fabrication of the compound semiconductor device.